Drug-eluting implantable medical devices such as stents have become popular in recent times for their ability to perform their primary function (such as structural support) and their ability to medically treat the area in which they are implanted.
For example, drug-eluting stents have been used to prevent restenosis in coronary arteries. Drug-eluting stents may administer biologically or pharmacologically active substances such as anti-inflammatory compounds that block local invasion/activation of monocytes, thus preventing the secretion of growth factors that may trigger VSMC proliferation and migration. Other potentially anti-restenotic compounds include anti-proliferative agents, such as chemotherapeutics, which include rapamycin and paclitaxel. Other classes of drugs such as anti-thrombotics, anti-oxidants, platelet aggregation inhibitors and cytostatic agents have also been suggested for anti-restenotic use.
Drug-eluting medical devices may be coated with a polymeric material which, in turn, is impregnated with a biologically or pharmacologically active substance or a combination of biologically or pharmacologically active substances. Once the medical device is implanted at a target location, the biologically or pharmacologically active substance is released from the polymer for treatment of the local tissues. The biologically or pharmacologically active substance is released by a process of diffusion through the polymer layer for biostable polymers, and/or as the polymer material degrades for biodegradable polymers.
Controlling the rate of elution of a biologically or pharmacologically active substance from the impregnated polymeric material is generally based on the properties of the polymer material. However, at the conclusion of the elution process, the remaining polymer material in some instances has been linked to an adverse reaction with the vessel, possibly causing a small but dangerous clot to form. Further, drug impregnated polymer coatings on exposed surfaces of medical devices may flake off or otherwise be damaged during delivery, thereby preventing the biologically or pharmacologically active substance from reaching the target site. Still further, drug impregnated polymer coatings are limited in the quantity of the biologically or pharmacologically active substance to be delivered by the amount of a drug that the polymer coating can carry and the size of the medical devices. Controlling the rate of elution using polymer coatings is also difficult.
Accordingly, stents with hollow, drug-filled structural members have also been contemplated. For example, U.S. Pat. No. 6,071,305 to Brown et al. generally discloses a stent formed of an elongated member in a spiral tube configuration. The elongated member includes a groove that can be filled with an active agent. Further, U.S. Application Publication No. 2011/0008405 to Birdsall et al. and U.S. Application Publication No. 2011/0070358 to Mauch et al., each of which is incorporated by reference herein in its entirety, describe methods of forming stents with hollow-drug-filled structural members from composite wires. However, preferred structural members for stents, such as nickel-titanium alloys (“nitinol”) and alloys of cobalt, nickel, chromium and molybdenum (“MP35N”, “MP20N”) are relatively radiolucent, especially when hollow. Thus, there is a need for a stent with hollow-drug filled structural members with improved radiopacity.